Sewing machines are known incorporating, upstream of the sewing needle, a device for controlling the tension of the sewing thread comprising at least two substantially flat plates, applied one against the other and between which the thread passes, a manually controlled device allowing tightening of the plates, and thus the thread, with a force of intensity characteristic of the tension desired at least on the portion of the thread extending downstream of the plates.
Such devices are not always entirely satisfactory, the thread naturally having a tendency to come out little by little from between the plates, until it is free from the device in question which then becomes inoperative. This displacement of the thread is due to the fact that, even though the thickness of such a thread is generally very small, of the order of several tenths of a mm to 1 mm for example, under the action of the tightening force to which they are subjected, the plates come into contact on the one hand on each other, at a point distant to the thread, and, on the other hand individually, on the thread itself. It follows that there is created on the thread a sort of "wedge" effect which is manifested by a force tending to push the thread in the opposite direction to the zone of mutual contact of the plates. When this force is superior to the frictional action of the plates on the thread, due to the tightening of these, the thread is displaced from between the plates until it is ejected.
Among the arrangements proposed to try to overcome this classic type of disadvantage, we refer to that disclosed in U.S. Pat. No. 4,123,984 (Singer) which proposes forming the periphery of each of the plates with two diametrically opposed indentations, arranged in such a way that the indentations of one plate are arranged in superimposed alignment with the corresponding indentations of the other plate, in an angular position relative to the plates, the pairs of superimposed indentations forming on the periphery of the assembly of the two plates, two distinct and concave lips (edges). In this device the thread is guided in such a way that it enters the plates across one of the lips, preferably near to one extremity of the lip in question, and that it leaves the device at a distance by the second lip.
This arrangement does not really give satisfaction, on one hand, because of certain types and dimensions of thread, and on the other hand, and even more so, in the case of sewing of simple designs, that is to say not requiring, for their formation, a significant density of thread per unit of surface, and moreover where the cloth for sewing is not too thick. In effect if, in the course of its passage in the zone of the plates close to the first edge thereof, the trajectory of the thread is very controlled for the reasons mentioned in the U.S. Pat. No. 4,123,984, this thread is more and more abandoned to itself in the course of its subsequent penetration between the plates so that the "wedge" effect hereinbefore mentioned rapidly becomes dominant and the thread acquires, here also, a natural tendency to leave the device.